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Versions: 00 01 02 03 04                                                
INTERNET DRAFT                                             Pat R. Calhoun
Category: Standards Track                          Sun Microsystems, Inc.
Title: draft-calhoun-diameter-proxy-01.txt                 William Bulley
Date: August 1998                                     Merit Network, Inc.



                               DIAMETER
                        Proxy Server Extensions



Status of this Memo

   This document is an individual contribution for consideration by the
   AAA Working Group of the Internet Engineering Task Force.  Comments
   should be submitted to the diameter@ipass.com mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at:

      http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at:

      http://www.ietf.org/shadow.html.


Abstract

   This DIAMETER Extension defines commands and AVPs that are used when
   DIAMETER messages must be proxied by DIAMETER Servers. This extension
   is intended to clearly define how proxying can be done with DIAMETER.






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Table of Contents

   1.0 Introduction
       1.1 Definitions
       1.2 Terminology
   2.0 Command Codes
       2.1 Domain-Discovery-Request (DDR)
       2.2 Domain-Discovery-Answer (DDA)
   3.0 DIAMETER AVPs
       3.1 Proxy-State
       3.2 Digital-Signature
       3.3 X509-Certificate
       3.4 X509-Certificate-URL
       3.5 Next-Hop
   4.0 Protocol Definition
       4.1 Data Integrity
           4.1.1 Using Digital Signatures
           4.1.1 Using Mixed Data Integrity AVPs
       4.2 AVP Data Encryption
           4.2.1 AVP Encryption with Public Keys
       4.3 Public Key Cryptography Support
           4.3.1 X509-Certificate
           4.3.2 X509-Certificate-URL
           4.3.3 Static Public Key Configuration
   5.0 References
   6.0 Acknowledgements
   7.0 Author's Address


1.0  Introduction

   Many services, including ROAMOPS and MobileIP, have a requirement for
   DIAMETER Server to proxy a request to another DIAMETER Server. The
   concept of proxying AAA requests was introduced by RADIUS and has
   been in use for many years.

   Unfortunately due to the fact that RADIUS only supports hop-by-hop
   security, where each node has a security association with the next
   hop, this does introduce some security flaws. Specifically a
   fraudulent proxy server can modify some portions of an AAA request in
   order to make the next hop improperly believe that some services were
   rendered. For example, a DIAMETER Proxy Server could modify an
   accounting request, such as the number of bytes that a user
   transfered, and the end system would have no way of determining that
   this change occured.


1.1  Definitions



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   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

   MUST      This word, or the adjective "required", means that the
             definition is an absolute requirement of the
             specification.

   MUST NOT  This phrase means that the definition is an absolute
             prohibition of the specification.

   SHOULD    This word, or the adjective "recommended", means that
             there may exist valid reasons in particular circumstances
             to ignore this item, but the full implications must be
             understood and carefully weighed before choosing a
             different course.

   MAY       This word, or the adjective "optional", means that this
             item is one of an allowed set of alternatives.  An
             implementation which does not include this option MUST
             be prepared to interoperate with another implementation
             which does include the option.


2.0  Command Codes

   This document defines the following DIAMETER Commands. All DIAMETER
   implementations supporting this extension MUST support all of the
   following commands:

      Command Name          Command Code
      -----------------------------------
      Domain-Discovery-Request  261
      Domain-Discovery-Answer   262


2.1  Domain-Discovery-Request (DDR)

   Description

      The Domain-Discovery-Request message is used by a DIAMETER device
      wishing to get contact information about a domain's home
      authentication server as well as to receive password policy
      information. This message MUST contain the User-Name attribute in
      order to pass along the user's domain information.

      It is not necessary for an implementation to issue a DDR in order
      to make use of a proxy server.




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      The X509-Certificate or the X509-Certificate-URL [2] MUST be
      present in this message in order to inform the home authentication
      server of the issuing host's certificate.

      At least one Extension-Id AVP MUST be present in the DDR in order
      to inform the peer about the locally supported extensions.

   Message Format

      <Domain-Discovery-Req> ::= <DIAMETER Header>
                                 <Domain-Discovery-Req Command AVP>
                                 <Host-IP-Address AVP>
                                 [<Host-Name AVP>]
                                 <Extension-Id AVPs>
                                 <User-Name AVP>
                                 [<X509-Certificate AVP>]
                                 [<X509-Certificate-URL AVP>]
                                 <Timestamp AVP>
                                 <Nonce AVP>
                                 {<Integrity-Check-Vector AVP> ||
                                  <Digital-Signature AVP }

   AVP Format

      A summary of the Domain-Discovery-Request packet format is shown
      below. The fields are transmitted from left to right.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |     Reserved      |P|T|V|E|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Command Code                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         256     DIAMETER Command

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending



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         upon the security model used. The 'V', 'T' and the 'P' bits
         MUST NOT be set.

      Command Type

         The Command Type field MUST be set to 261 (Domain-Discovery-
         Request).


2.2  Domain-Discovery-Answer (DDA)

   Description

      The Domain-Discovery-Answer message is sent in response to the
      Domain-Discovery-Request message by the domain's Home
      authentication server. The message MUST contain either the Host-
      Name or Host-IP-Address and either the X509-Certificate or the
      X509-Certificate-URL attribute and SHOULD contain at least one
      Framed-Password-Policy AVP.

      At least one Extension-Id AVP MUST be present in the DDA in order
      to inform the requestor about the locally supported extensions.

      The Domain-Discovery-Answer message MUST include the Result-Code
      AVP to indicate whether the request was successful or not. The
      following Error Codes are defined for this command:

         DIAMETER_ERROR_UNKNOWN_DOMAIN       1
            This error code is used to indicate to the initiator of the
            request that the requested domain is unknown and cannot be
            resolved.

         DIAMETER_ERROR_BAD_CERT             2
            This error code is used to indicate that the X509-
            Certificate or the X509-Certificate-URL in the Domain-
            Discovery-Request was invalid.

         DIAMETER_ERROR_CANNOT_REPLY         3
            This error code is returned when either an intermediate
            DIAMETER node or the home authentication server cannot reply
            to DIAMETER messages directly.  This could be that the
            policy of an intermediate DIAMETER server does not permit
            direct contact and therefore requires proxying. It could
            also signify that the home authentication server does not
            have public key support.

   Message Format




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      <Domain-Discovery-Answer> ::= <DIAMETER Header>
                                    <Domain-Disc-Answer Command AVP>
                                    <Result-Code AVP>
                                    [<Error-Code AVP>]
                                    <Host-IP-Address AVP>
                                    [<Host-Name AVP>]
                                    <Extension-Id AVPs>
                                    <Framed-Password-Policy AVP>
                                    [<X509-Certificate AVP>]
                                    [<X509-Certificate-URL AVP>]
                                    <Timestamp AVP>
                                    <Nonce AVP>
                                    {<Integrity-Check-Vector AVP> ||
                                     <Digital-Signature AVP }

   AVP Format

      A summary of the Domain-Discovery-Answer packet format is shown
      below. The fields are transmitted from left to right.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |     Reserved      |P|T|V|E|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Command Code                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         256     DIAMETER Command

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
         upon the security model used. The 'V', 'T' and the 'P' bits
         MUST NOT be set.

      Command Type

         The Command Type field MUST be set to 262 (Domain-Discovery-
         Answer).



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3.0  DIAMETER AVPs

   This section will define the mandatory AVPs which MUST be supported
   by all DIAMETER implementations. Note the first 256 AVP numbers are
   reserved for RADIUS compatibility.

   The following AVPs are defined in this document:

      Attribute Name       Attribute Code
      -----------------------------------
      Proxy-State                33
      Digital-Signature         260
      X509-Certificate          264
      X509-Certificate-URL      265
      Next-Hop                  278


3.1  Proxy-State

   Description

      The Proxy-State AVP is used by proxy servers when forwarding
      requests and contains opaque data that is used by the proxy to
      further process the response.

      This attribute should be removed by the proxy server before the
      response is forwarded to the NAS, and SHOULD therefore not be
      protected by the Integrity-Check-Vector or the Digital-Signature.

   AVP Format

      A summary of the Proxy-State AVP format is shown below. The fields
      are transmitted from left to right.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |     Reserved      |P|T|V|E|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Data ...
      +-+-+-+-+-+-+-+-+

      AVP Code

         33 for Proxy-State.




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      AVP Length

         The length of this attribute MUST be at least 9.

      AVP Flags

         The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
         upon the security model used. The 'V', 'T' and the 'P' bits
         MUST NOT be set.

      String

         The String field is one or more octets. The actual format of
         the information is site or application specific, and a robust
         implementation SHOULD support the field as undistinguished
         octets.


3.2  Digital-Signature

   Description

      The Digital-Signature AVP is used for authentication, integrity as
      well as non-repudiation. A DIAMETER entity adding AVPs to a
      message MUST ensure that all AVPs appear prior to the Digital-
      Signature AVP (with the exception of the Integrity-Check-Vector
      AVP that MUST appear after the Digital-Signature AVP). The
      Timestamp AVP MUST be present to provide replay protection and the
      Nonce AVP must be present to add randomness to the packet.

      The DIAMETER header as well as all AVPs with the 'P' bit disabled
      are protected by the Digital-Signature.

      In order to support proxy DIAMETER servers, which forwards
      messages to next hop server, the proxy server MUST NOT modify any
      AVPs with the 'P' bit disabled. This ensures that end-to-end
      security is maintained even through proxy arrangements.

      The Digital-Signature is generated in the method described in
      section 4.5.2.

      All DIAMETER implementations supporting this extension MUST
      support this AVP.

   AVP Format

      A summary of the Digital-Signature AVP format is shown below. The
      fields are transmitted from left to right.



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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |     Reserved      |P|T|V|E|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            Address                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Transform ID                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Data ...
      +-+-+-+-+-+-+-+-+

      AVP Code

         260     Digital-Signature

      AVP Length

         The length of this attribute MUST be at least 17.

      AVP Flags

         The 'M' bit MUST be set. The 'H' MAY be set if the request is
         protected with an ICV AVP. The 'E', 'V', 'T' and the 'P' bits
         MUST NOT be set.

      Address

         The Address field contains the IP address of the DIAMETER host
         which generated the Digital-Signature.

      Transform ID

         The Transform ID field contains a value that identifies the
         transform that was used to compute the signature. The following
         values are defined in this document:

         RSA [9]          1

      Data

         The Data field contains the digital signature of the packet up
         to this AVP.


3.3  X509-Certificate



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   Description

      The X509-Certificate is used in order to send a DIAMETER peer the
      local system's X.509 certificate chain, which is used in order to
      validate the Digital-Signature attribute.

      Section 4.3.1 contains more information about the use of
      certificates.

   AVP Format

      A summary of the X509-Certificate AVP format is shown below. The
      fields are transmitted from left to right.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |     Reserved      |P|T|V|E|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Data ...
      +-+-+-+-+-+-+-+-+

      AVP Code

         264     X509-Certificate

      AVP Length

         The length of this attribute MUST be at least 9.

      AVP Flags

         The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
         upon the security model used. The 'V', 'T' and the 'P' bits
         MUST NOT be set.

      Data

         The Data field contains the X.509 Certificate Chain.


3.4  X509-Certificate-URL

   Description

      The X509-Certificate-URL is used in order to send a DIAMETER peer



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      a URL to the local system's X.509 certificate chain, which is used
      in order to validate the Digital-Signature attribute.

      Section 4.3.1 contains more information about the use of
      certificates.

   AVP Format

      A summary of the X509-Certificate-URL AVP format is shown below.
      The fields are transmitted from left to right.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |     Reserved      |P|T|V|E|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   String ...
      +-+-+-+-+-+-+-+-+

      AVP Code

         265     X509-Certificate-URL

      AVP Length

         The length of this attribute MUST be at least 9.

      AVP Flags

         The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
         upon the security model used. The 'V', 'T' and the 'P' bits
         MUST NOT be set.

      String

         The String field contains the X.509 Certificate Chain URL.


3.5  Next-Hop

   Description

      The Next-Hop AVP MUST preceed a Digital-Signature AVP and is used
      to validate that a packet traversed the proxy chain that was
      intended. A DIAMETER message that includes a Host-IP-Address with
      a different Next-Hop AVP that preceeds it is considered invalid.



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   AVP Format

      A summary of the Next-Hop AVP format is shown below. The fields
      are transmitted from left to right.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |     Reserved      |P|T|V|E|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            Address                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         278     Next-Hop

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
         upon the security model used. The 'V', 'T' and the 'P' bits
         MUST NOT be set.

      Address

         This field contains the IP Address of the next DIAMETER Server.


4.0  Protocol Definition

   This section will describe how the base protocol works (or is at
   least an attempt to).


4.1  DIAMETER Proxying

   The ROAMOPS specification [11] discusses how RADIUS servers can be
   arranged in a hierarchical manner, allowing message exchanges across
   domain boundaries. The specification also describes some security
   flaws encountered when RADIUS is used in a proxy environment. The
   DIAMETER extension described in this document introduces end-to-end
   security, which solves many of the problems encountered when RADIUS



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   is used.

                   (Request)                   (Request)
      +------+       ----->       +------+      ------>       +------+
      |      |                    |      |                    |      |
      | NASB +--------------------+ DIA2 +--------------------+ DIA1 |
      |      |                    |      |                    |      |
      +------+       <-----       +------+      <------       +------+
                     (Answer)                   (Answer)

   In this example NASB generates a Request that is forwarded to DIA2.
   The Request contains a digital signature AVP which "protects" all
   mandatory (or non-editable) AVPs within the request. All AVPs which
   may be modified, or removed appear after the digital signature AVP.
   Once DIA2 receives the request, it MAY authenticate the request to
   ensure that it was originated by NASB (verifying the signature is not
   necessary if the link between NASB and DIA2 is secured using IPSEC).

   The DIA2 Server SHOULD add the Proxy-State AVP, which contains opaque
   data that MUST be present in the response and is used to identify
   state information related to the request or response. If the Proxy-
   State AVP is already present in the request, it MUST be replaced with
   DIA2's Proxy-State AVP. This means that the Proxy-State AVP cannot be
   protected end-to-end. The Server MAY also add other new AVPs to the
   request. All AVPs that are protected by the Digital-Signature MUST
   have the 'P' bit set, and all AVPs MUST preceed the Digital-Signature
   AVP. The message is then forwarded towards the DIA1 server.

   Since all packets between NASB and DIA1 must flow through DIA2, it is
   not possible to use IPSEC between them, therefore the digital
   signature must be present within the DIAMETER protocol itself. NASB
   and DIA1 do not need to validate the digital signature AVP if the
   link between themselves and DIA2 is secured using IP Security.

   This mechanism now provides a method for DIA1 to prove that NASB was
   the initiator of the request (note that DIAMETER also includes a
   timestap to prevent replay attacks). It also provides a method of
   ensuring that DIA2 cannot modify any protected AVPs (such as length
   of call, etc.).

   In addition, this same mechanism can be used for end-to-end
   encryption of AVPs. In the case where NASB needs to encrypt an AVP it
   is done using asymetric encryption using DIA1's public key. This
   ensures that only DIA1 can decrypt the AVP.

   An attack has been identified in this proposal which allows a
   malicous man in the middle attack as shown in the following diagram.




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              (Request)         (Request)         (Request)
      +------+  ----->  +------+  ----->  +------+  ----->  +------+
      |      |          |      |          |      |          |      |
      | NASB +----------+ DIA2 +----------+ DIA3 +----------+ DIA1 |
      |      |          |      |          |      |          |      |
      +------+  <-----  +------+  <-----  +------+  <-----  +------+
                (Answer)         (Answer)          (Answer)

   In this example, DIA3 traps packets generated from DIA2 towards DIA1,
   removes the AVPs added by DIA2 and inserts its own AVPs (possibly by
   trying to convince DIA1 to pay DIA3 for the services). This attack
   can be prevented by supporting a new Next-Hop AVP. In this case when
   NASB prepares a request it inserts a non-editable Next-Hop AVP which
   contains DIA2's identitity. DIA2 also adds a Next-Hop AVP with DIA1
   as the next hop.

   This mechanism ensures that a man in the middle cannot alter the
   packet by overriding the previous hop's additions and signature. DIA1
   could easily validate the packet's path with the use of the Next-Hop
   AVPs.


4.2  Domain Discovery

   The Domain Discovery message set is very useful in determining the
   Home authentication server, the password policies for the domain, as
   a mechanism to retrieve a certificate (or a pointer to a
   certificate).

   Note that it is not necessary for a host to issue a Domain Discovery
   in order to make use of a proxy. A DIAMETER Request MAY be proxied by
   an intermediate server without the knowledge of the client, however
   the client will be unable to validate any Digital-Signatures if the
   home authentication server's certificate or public key is not known.

   The following example shows a case where DIA1 needs to communicate
   with DIA3. In this example it is necessary to use DIA2 as a proxy in
   order for both ISPs to communicate. Although this MAY be desireable
   in some business models, there are cases where it is beneficial to
   remove the proxy altogether and allow both DIA3 and DIA1 to
   communicate in a secure fashion.










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                  (DD-Request)               (DD-Request)
      +------+       ----->       +------+      ------>       +------+
      |      |                    |      |                    |      |
      | DIA1 +--------------------+ DIA2 +--------------------+ DIA3 |
      |      |                    |      |                    |      |
      +------+       <-----       +------+      <------       +------+
                  (DD-Response)              (DD-Response)

   The way the Domain Discovery works is that prior to sending out an
   authentication request DIA1 would issue a Domain Discovery message
   towards DIA2. This message is protected with the digital signature as
   well as the Next-Hop AVP. DIA2 would then forward the request to DIA3
   including the Next-Hop and the digital signature AVP.

   When DIA3 receives the request, it MUST save the certificate (or the
   pointer to the certificate) and respond back including the local
   password policy, DIA3's certificate, its contact information (i.e. IP
   address) and protect the response with the digital signature.

   Note that in all cases the TimeStamp AVP is also present to ensure no
   replay packets are accepted.

   When DIA2 receives the packet, it must add the Next-Hop AVP as well
   as the digital signature AVP. When DIA1 receives the packet it then
   knows a direct route to communicate with DIA3 since the contact
   information is present in the response. The fact that both DIA1 and
   DIA3 can now communicate directly allows both peers to use IPSEC to
   protect the message exchange (it may be desirable to use the
   Digital-Signature AVP in instances where records of digitally signed
   packets must be kept).

                                  (Request)
                     +------+       ----->       +------+
                     |      |                    |      |
                     | DIA1 +--------------------+ DIA3 |
                     |      |                    |      |
                     +------+       <-----       +------+
                                   (Answer)

   In addition, the password policy is also present which can indicate
   whether DIA3 is willing to accept CHAP, PAP or EAP authentication.

   Note that the Domain-Discovery-Request/Answer MUST include at least
   one Extension-Id AVP [2].


4.3  Data Integrity




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   This section will describe how data integrity and non-repudiation is
   achieved using the Digital-Signature AVP.

   Note that the Timestamp and Nonce AVPs MUST be present in the message
   PRIOR to the Digital-Signature AVPs discussed in this section. The
   Timestamp AVP provides replay protection and the Nonce AVP provides
   randomness.

   Any AVPs in a message that is not followed by either the ICV or the
   Digital-Signature AVPs MUST be ignored.


4.3.1  Using Digital Signatures

   In the case of a simple peer to peer relationship the use of IPSEC is
   sufficient for data integrity and non-repudiation. However there are
   instances where a peer must communicate with another peer through the
   use of a proxy server. IPSEC does not provide a mechanism to protect
   traffic when two peers must use an intermediary node to communicate
   at the application layer therefore the Digital-Signature AVP MUST be
   used.

   The following diagram shows an example of a router initiating a
   DIAMETER message to DIA1. Once DIA1 has finished processing the
   message it adds its signature and forwards the message to the non-
   trusted DIA2 proxy server. If DIA2 needs to add or change any
   protected AVPs it SHOULD add its digital signature before forwarding
   the message to DIA3.

      +------+  ----->  +------+  ----->  +------+  ----->  +------+
      |      |          |      |          | non- |          |      |
      |router+----------+ DIA1 +----------+trustd+----------+ DIA3 |
      |      |          |      |          | DIA2 |          |      |
      +------+  <-----  +------+  <-----  +------+  <-----  +------+

   Since some fields within the DIAMETER header will change "en route"
   towards the final DIAMETER destination, it is necessary to set the
   unprotected fields to zero (0) prior to calculating the signature.
   The two unprotected fields are the identifier and the length in the
   DIAMETER header.

   The following is an example of a message that include end-to-end
   security:








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      <DIAMETER Message> ::= <DIAMETER Header>
                             <Command AVP>
                             [<Additional AVPs>]
                             <Next-Hop AVP>
                             <Timestamp AVP>
                             <Nonce AVP>
                             <Digital-Signature AVP>

   The AVP Header's 'P' bit is used to identify which AVPs are
   considered protected when applying a digital signature to a DIAMETER
   message. Protected AVPs cannot be changed "en route" since they are
   protected by the Digital Signature AVP. All AVPs added by a DIAMETER
   entity MUST appear prior to the Digital Signature AVP that is added
   (with the exception of the Integrity-Check-Vector AVP). However, only
   AVPs with the 'P' bit set are used in the digital signature
   calculation.

   The Next-Hop AVP indicates the intended recipient of the DIAMETER
   message.  When a DIAMETER message is received with a Next-Hop AVP
   that does not correspond with the Host-IP-Address that follows, the
   message MUST be considered invalid and MUST be rejected.

   The Data field of the Digital-Signature AVP contains the RSA/MD5
   signature algorithm as defined in [9].


4.3.2  Using Mixed Data Integrity AVPs

   The previous sections described the Integrity-Check-Vector and the
   Digital-Signature AVP. Since the ICV offers hop-by-hop integrity and
   the digital signature offers end to end integrity, it is possible to
   use both AVPs within a single DIAMETER message.

   The following diagram provides an example where DIAMETER Server 1
   (DIA1) communicates with DIA3 using Digital-Signatures through DIA2.
   In this example ICVs are used between DIA1 and DIA2 as well as
   between DIA2 and DIA3.

                      <Public-Key>
             ----------------------------->
            <Shared-Secret>   <Shared-Secret>
      +------+  ----->  +------+  ----->  +------+
      |      |          |      |          |      |
      | DIA1 +----------+ DIA2 +----------+ DIA3 |
      |      |          |      |          |      |
      +------+          +------+          +------+

   Using the previous diagram, the following message would be sent



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   between DIA1 and DIA2:

      <DIAMETER Message> ::= <DIAMETER Header>
                             <Command AVP>
                             [<Additional AVPs>]
                             <Timestamp AVP>
                             <Nonce AVP>
                             <Digital-Signature AVP>
                             <Integrity-Check-Vector AVP (DIA1->DIA2)>

   The following message would be sent between DIA2 and DIA3:

      <DIAMETER Message> ::= <DIAMETER Header>
                             <Command AVP>
                             [<Additional AVPs>]
                             <Timestamp AVP>
                             <Nonce AVP>
                             <Digital-Signature AVP>
                             <Timestamp AVP>
                             <Nonce AVP>
                             <Integrity-Check-Vector AVP (DIA2->DIA3)>

   Note that in the above example messages the ICV AVP appear after the
   Digital-Signature AVP. This is necessary since DIA2 above removes the
   ICV AVP (DIA1->DIA2) and adds its own ICV AVP (DIA2->DIA3). The ICVs
   provide hop-by-hop security while the Digital-Signature provides
   integrity of the message between DIA1 and DIA3.

            <Shared-Secret>    <Public-Key>
      +------+  ----->  +------+  ----->  +------+
      |      |          |      |          |      |
      |router+----------+ DIA1 +----------+ DIA2 |
      |      |          |      |          |      |
      +------+  <-----  +------+  <-----  +------+

   There are cases, such as in remote access, where the device
   initiating the DIAMETER request does not have the processing power to
   generate Digital-Signatures as required by the protocol. In such an
   arrangement, there normally exists a first hop DIAMETER Server (DIA1)
   which acts as a proxy to relay the request to the final
   authenticating DIAMETER server (DIA2). It is valid for the first hop
   server to remove the Integrity-Check-Vector AVP inserted by the
   router and replace it with a Digital-Signature AVP.


4.4  AVP Encryption with Public Keys

   AVP encryption using public keys is much more complex than the



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   previously decribed method, yet it is desirable to use it in cases
   where the DIAMETER message will be processed by an untrusted
   intermediate node (proxy).

   Public Key encryption SHOULD be supported, however it is permissible
   for a low powered device initiating the DIAMETER message to use
   shared secret encryption with the first hop (proxy) DIAMETER server,
   which would decrypt and encrypt using the Public Key method.

   The PK-Encrypted-Data bit MUST only be set if the final DIAMETER host
   is aware of the sender's public key. This information can be relayed
   in three different methods as described in section 4.3.

   The AVP is encrypted in the method described in [9].


4.5  Public Key Cryptography Support

   A DIAMETER peer's public key is required in order to validate a
   message which includes the the Digital-Signature AVP. There are three
   possibilities on retrieving public keys:

4.5.1  X509-Certificate

   A message which includes a Digital-Signature MAY include the X509-
   Certificate AVP. Given the size of a typical certificate, this is
   very wasteful and in most cases DIAMETER peers would cache such
   information in order to minimize per packet processing overhead.

   It is however valid for a DIAMETER host to provide its X509-
   Certificate in certain cases, such as when issuing the Device-
   Reboot-Indication. It is envisioned that the peer would validate and
   cache the certificate at that time.


4.5.2  X509-Certificate-URL

   The X509-Certificate-URL is a method for a DIAMETER host sending a
   message that includes the Digital-Signature to provide a pointer to
   its certificate.

   Upon receiving such a message a DIAMETER host MAY choose to retrieve
   the certificate if it is not locally cached. Of course the process of
   retrieving and validating a certificate is lengthy and will require
   the initiator of the message to retransmit the request. However once
   cached the certificate can be used until it expires.





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4.5.3  Static Public Key Configuration

   Given that using certificates requires a PKI infrastructure which is
   very costly, it is also possible to use this technology by locally
   configuring DIAMETER peers' public keys.

   Note that in a network involving many DIAMETER proxies this may not
   scale well.


5.0  References

    [1] Rigney, et alia, "RADIUS", RFC-2138, April 1997
    [2] Calhoun, Rubens, "DIAMETER Base Protocol", Internet-Draft,
        draft-calhoun-diameter-05.txt, August 1998.
    [3] Rivest, Dusse, "The MD5 Message-Digest Algorithm",
        RFC 1321, April 1992.
    [4] Kaufman, Perlman, Speciner, "Network Security: Private
        Communications in a Public World", Prentice Hall,
        March 1995, ISBN 0-13-061466-1.
    [5] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for Message
        Authentication", RFC 2104, January 1997.
    [6] Calhoun, Bulley, "DIAMETER User Authentication Extensions",
        draft-calhoun-diameter-authen-03.txt, May 1998.
    [7] Aboba, Beadles "The Network Access Identifier." RFC 2486.
        January 1999.
    [8] Aboba, Zorn, "Criteria for Evaluating Roaming Protocols",
        RFC 2477, January 1999.
    [9] Kaliski, "PKCS #1: RSA Encryption Version 1.5", Internet-
        Draft, draft-hoffman-pkcs-rsa-encrypt-03.txt, October 1997.
    [10] Calhoun, Zorn, Pan, "DIAMETER Framework",
         draft-calhoun-diameter-framework-02.txt, Work in Progress,
         December 1998
    [11] Aboba, Vollbrecht, "Proxy Chaining and Policy Implementation in
         Roaming", draft-ietf-roamops-auth-10.txt, Work in Progress,
         February 1999.

6.0  Acknowledgements

   The Authors would like to acknowledge the following people for their
   contribution in the development of the DIAMETER protocol:

   Bernard Aboba, Jari Arkko, , Daniel C. Fox, Lol Grant, Nancy Greene,
   Peter Heitman, Ryan Moats, Victor Muslin, Kenneth Peirce, Allan
   Rubens, Sumit Vakil, John R. Vollbrecht, Jeff Weisberg and Glen Zorn


7.0  Author's Address



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   Questions about this memo can be directed to:

      Pat R. Calhoun
      Technology Development
      Sun Microsystems, Inc.
      15 Network Circle
      Menlo Park, California, 94025
      USA

       Phone:  1-650-786-7733
         Fax:  1-650-786-6445
      E-mail:  pcalhoun@eng.sun.com


      William Bulley
      Merit Network, Inc.
      4251 Plymouth Road, Suite C
      Ann Arbor, Michigan, 48105-2785
      USA

       Phone:  1-734-764-9993
         Fax:  1-734-647-3185
      E-mail:  web@merit.edu




























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